u can cut a spare axle or u can use a picture nail (its about the same size). i tried everything from a paper clip (industrial 1's), matchbox, nails, etc. but i ended up using the axle from the hot wheels kit racer.
i did mine
by taking the bit axel
and cutting the wheels off
and leaving the gear in place
then you have a good axel
just glue the wheels on
check out my bb a few threads back